The present invention relates to high vacuum turbomolecular pumps and, more particularly, to such a pump having an improved lubrication arrangement for its rotor bearings.
Turbomolecular pumps are increasingly being used to pump in the free molecule pressure range, i.e., 10.sup.-3 to 10.sup.-10 torr, because they are inherently non-contaminating. They operate on kinetic gas principles and rely on the relative motion between gas molecules and an interleaved cascade of stator and rotor blade wheels upon which they impact. The design of the blade wheels makes it more probable that a molecule striking the same will rebound toward the pump outlet than toward its inlet.
In general the pumping speed and efficiency of a turbomolecular pump is dependent upon the speed of rotation of the rotor blades. For this reason, rotor speeds in the high range of 35,000 to 50,000 r.p.m. are not unusual. Such high rotational speeds place relatively stringent structural limits and lubrication demands on the rotor. To assure balanced rotation, the rotors of most modern pumps are arranged to rotate on a vertical axis, and the motor responsible for such high rotation typically is an induction-type motor incorporated directly into the pump with the motor rotor output shaft also serving as the axle or, in other words, spindle for the pump rotor.
It will be appreciated that with such high speed operation, the amount and type of lubrication of the bearings supporting the spindle and, hence, supporting the turbine rotor can measurably affect the efficiency of the pump. In this connection, because the motor typically operates in a reduced pressure atmosphere, the amount of heat generated by the same must be kept at a minimum by optimizing the efficiency of its operation, e.g., reducing bearing drag, etc.
U.S. Pat. No. 3,877,546 describes a typical lubrication system for a turbomolecular pump having a rotor which rotates on a vertical axis. As illustrated therein, the pump is provided at its bottom with a reservoir of lubricating oil within which the lower end of the rotor spindle is immersed. An axial passageway extends upwardly through the spindle from such lower end to a location adjacent (slightly above) an upper bearing supporting the spindle for rotation. On rotation of the spindle, oil from the reservoir is drawn centrifugally up the passageway and is directed by radial outlets at the top of the spindle to the upper bearing for lubricating the same. A lower bearing for the spindle is lubricated by collecting the oil flowing downwardly from the upper bearing and feeding the same to the lower bearing prior to such oil being returned to the reservoir.
While lubrication systems of the general type described are considered the most acceptable by many, there are difficulties associated with the same, particularly at the immersed lower end of the spindle. For one, as the spindle rotates at a high r.p.m., it causes turbulence within the reservoir at the inlet orifice to the spindle, which turbulence prevents a continuous controlled flow into the spindle shaft. This problem is compounded by operation at the reduced atmospheric pressure levels typical of the reservoir section of turbomolecular pumps. When the vacuum level in such section approaches the vapor pressure of the lubricant, adverse velocity and pressure gradients caused by the turbulence can cause cavitation (local vaporization) of the oil. This can cause either intermittent or continuous stoppage of oil flow into the shaft, depending on factors such as the shaft speed, oil properties, vacuum level, etc.
In an effort to reduce the affect of turbulence on the entry of oil into the shaft passageway, it has been the practice to reduce the shaft surface area immediately adjacent the passageway orifice. That is, the end of the shaft is typically machined or otherwise reduced in diameter, often to be a knife edge in cross section at the inlet orifice. While such a geometrical configuration tends to reduce the problems of turbulence immediately adjacent the inlet orifice, it does not eliminate the same, nor assure a continuous, controlled amount of flow into the passageway.
Another problem associated with immersion of the lower end of the spindle is that as the spindle rotates frictional drag caused by the lubricant results in shaft power loss. This power loss appears as heat generation in the motor which, as mentioned previously, should be avoided.